1. Field of the Invention
This invention relates to a head tracker system and more especially, although not exclusively, to a head tracker system for use in an aircraft in which the pilot is provided with a helmet-mounted display.
2. Discussion of Prior Art
Head tracker systems are well known and operate to determine a user's head orientation and position relative to a fixed datum. Originally these systems were developed for use in military aviation but have recently found applications in virtual reality systems.
Since the earliest days of military aviation, pilots have not surprisingly preferred to navigate and aim weapons whilst looking up and out of the cockpit. This led to the evolution of the head-up display (HUD) which displays useful symbology which is appropriately referenced to the outside world (often termed “space stabilised”). HUDs typically have a viewing angle of thirty degrees or less and can consequently only be viewed when the pilot is looking in a generally forward direction. To increase the field of regard (that is the total volume of space over which the pilot can view the symbology and includes the pilot moving his head), helmet-mounted displays (HMDs) have evolved which essentially comprise a HUD which is mounted on the pilot's helmet within his field of view. In order that ground stabilized symbols or imagery are presented to the pilot in the correct orientation with respect to the outside world, the symbol generator of the HUD must know in all three axes, that is elevation, azimuth and roll, where the pilot's head is directed. This is achieved by determining (a) the angular orientation of the pilot's head with respect to the aircraft axes and (b) the orientation (attitude) of the aircraft with the outside world. The former requirement has led to the need for a head tracker system. HMDs operate in conjunction with the head tracker system which determines the angular orientation of the user's head with respect to the aircraft axes to ensure that the displayed information is correctly aligned in space or is accurately superimposed against objects in the outside world. For example, in the case of military aviation it is essential that a weapon aiming marker is accurately aligned over the target.
Typically, a head tracker system comprises a head mounting, most commonly a helmet, which is attached to the user's head and a sensor system for determining the angular orientation of the helmet relative to a fixed reference datum. Although strictly speaking a head tracker system actually tracks the orientation of the head mounting and not the user's head, it provides an accurate measure of the user's head orientation provided the head mounting remains in a fixed orientation relative to the user's head.
The reference datum is typically three axes which pass through a known point on the vehicle or airframe. A number of sensor systems have been proposed. For example, early tracking systems used mechanical linkages between the helmet and the vehicle and, whilst such systems were relatively accurate, they were cumbersome, restricted the user's movement and posed particular problems during ejection from an aircraft. Further systems include optical sensor systems in which the helmet carries a number of light emitting diodes operating in the infra-red which are detected by position sensitive detectors which are mounted to the vehicle cockpit. Other known optical systems use visually distinctive physical markings which are detected by a camera. Magnetic sensor systems are also known in which an alternating or pulsed magnetic field is detected by sensors on the helmet. Inertial systems have been proposed which use gyroscopes and accelerometers; and hybrid systems which involve a combination of two or more of the above systems.
A particular requirement of a head tracker system for use in military aviation is high accuracy coupled with a fast dynamic response as the pilot's head movements are often extremely rapid; typically these can be greater than two hundred degrees per second. These two requirements are often mutually exclusive and the known tracker systems are a compromise in achieving these objectives. For example, although inertial systems based on gyroscopes have a very fast dynamic response, the accuracy of these systems is limited by drift over extended periods of operation. Tracker systems which are based on magnetic sensors, although accurate, do not provide a fast enough dynamic response because of the slow settling times of the magnetic fields. Although inherently fast enough, the accuracy of optical systems is constrained by the large amount of electronic processing required. The latency in data processing in most tracker systems is further compounded by the additional lag introduced by the filtering needed to minimise the effect of noise on the often very low levels of signals being detected. In an attempt to overcome these problems it has been proposed to have tracker systems which are hybrid systems and which involve a combination of two of the above systems such as, for example, an inertial system to provide the dynamic response which is supplemented by a magnetic system for long term accuracy.
The present invention has arisen in an endeavour to provide a system which, at least in part, overcomes the limitations of the known head tracker systems.